Hollow filaments of a tetrafluoroethylene polymer and process for preparing them



March 5, 1968 YUTAKA KOMETANI ETAL 3,372,082

HOLLOW FILAMENTS OF A TETRAFLUOROETHYLENE POLYMER AND PROCESS FORPREPARING THEM Filed Dec. 2, 1964 United StatesPatent fice 3,372,082Patented Mar. 5, 1968 3,372,082 HOLLOW FILAMEN'IS 0F A TETRAFLUORO-ETHYLENE POLYMER AND PROCESS FOR PREPARING THEM Yutaka Kometani,Hyoga-ken, Yutaka Kuda, Osaka-fu, Tatiahiko Sugino, Osaka-shi, andHirokazu Izumi, Amagasaki-slxi, Japan, assignors to Daikin KogyoKabushiki Kaisha, Osaka-shi, Japan Filed Dec. 2, 1964, Ser. No. 415,327Claims priority, application Japan, Dec. 6, 1963, 38/ 65,774 4 Claims.(Cl. 161-178) ABSTRACT F THE DISCLOSURE Hollow filaments consisting of atetrafluoroethylene polymer layer and an empty space cavity Within, saidempty space cavity comprising from 5 to 90 percent in terms of SbSufi-Sb wherein Sa is the total area of the tetrafiuoroethylene polymerlayer at a given cross-section of said hollow filament, and Sb is thetotal area of the empty space cavity formed therein at said givensection produced by extruding a mixture of a viscose and an aqueousdispersion of a tetrafluoroethylene polymer at a temperature rangingfrom 50 to 100 C. into a setting medium selected from the groupconsisting of a bath containing sulfuric acid in the order of from 6 to60 percent by weight, a bath containing sulfuric acid in the order of atmost 6 percent by weight and a sulfuric acid salt in the order of atleast 5 percent by weight, a bath containing hydrochloric acid in theorder of from l to 3() percent by weight, and a bath containing nitricacid in the order of from l to 50 percent by weight, and heating theresultant filament product to coalcsce the tetrafluoroethylene polymerforming said filament product, said starting spinning mixture comprisingcellulose in the order from 0.4 to 8 percent by weight and atetrafiuoroethylene polymer in the order of from 70 to 99 percent byweight 0f the combined weight of said cellulose and polymer.

This invention relates to hollow filaments or fibers oftetrafluoroethylene polymers. The invention further pertains to themanufacture of said filaments having an empty space or cavity within thebody.

Tetraiiuoroethylene polymers have a high order of stability to heat andchemicals, which properties make them very useful for various purposes.But their inertness to melting or dissolving necessitates to apply aspecific procedure for the shaping of fibrous structures therefrom. Asdisclosed in British Patent 767,015 and United States Patent 2,772,444,the desired filaments are generally produced by emulsion spinning from amixture of la matrix-forming material and an aqueous dispersion of atetrafluoroethylene polymer. Such spinning procedure, however, iscarried out under the conditions which are confined to the production offilaments all ha'ving the body stuffed or filled up with the startingtetraiiuoroethylene polymer. In other words, there has not yet beenreported in the prior art either the provision of hollow filaments oftetrafiuoroethylene polymers having an empty space or cavity within thebody or a process for the manufacture of said hollow filaments fromtetraiiuoroethylene polymers.

The inertness to melting or dissolving of said tetrafiuoroethylenepolymers, further, precludes the application thereto of the knownmethods for the manufacture of filaments or fibers having hollowstructures out of such polymers as polyester, polyvinyl alcohol,polyvinyl chloride or polyamide which can be easily melted or dissolved.According to such methods, the desired hollow filaments are generallyproduced by melt spinning comprising extruding a fused polymer from aspinneret having a specific structure, or by solution spinningcomprising extruding a polymer solution containing an additive, such asvolatile or frothing agent, into a setting medium, allowing the additiveemployed to evaporate or froth in the course of the spinning procedure.

It is accordingly one object of this invention to provide hollowfilaments having an empty space cavity within the body fromtetrafluoroethylene polymers.

Another object of the invention `is the provision of hollow filamentsexcelled in a high order of bulkiness and a low order of specificgravity resulting from the aforesaid empty space cavity within the bodyand having a high order of insulation to heat and of compressiveelasticity.

A further object of the invention is the provision of hollow filamentspronounced in a reduction in cost per unit due to the -aforesaidcharacters.

A still further object of the invention is the provision of a processfor the manufacture of the hollow filaments having the aforesaidcharacters from tetraiiuoroethylene polymers.

A still another object of the invention is the provision of a processfor converting almost all amount of the tetrafiuoroethylene polymersemployed into the desired hollow filaments.

A still further object of the invention is the provision of a processfor the manufacture of hollow filaments out of tetrafiuoroethylenepolymers which can be carried out without the use of volatile orfrothing agent. A

Other objects and specific features of this invention will becomeapparent in view of the subsequent description.

According to the principles of this invention, the hollow filaments ofthe same consist of a body layer of the tetrafiuoroethylene polymeremployed and an empty space cavity within the body in the range of from5 to percent at a given cross section. The term empty space cavity asemployed herein is defined as below:

Sa+S b X wherein Sa is the total area of the tetrafluoro-ethylenepolymer layer at a given cross section of the hollow filaments examined,and Sb is the total empty area at said cross section of the hollowfilaments examined.

In order to afford a fuller understanding of this invention, there areappended illustrative drawings, in which:

FIGURE 1 is an enlarged cross sectional view of a hollow filament ofthis invention;

FIGURE 2 is an enlarged cross sectional view of a hollow filament-bundle of this invention; and

FIGURE 3 is an enlarged cross sectional view of a conventional fibrousbundle shown as a comparison.

As will be apparently seen from the drawings, the spun filaments of thisinvention respectively consist of a polymer layer part 1 and an internalcavity or empty space part 2 formed within the body of the filament. Thepolymer layer part 1 is filled up with a continuous layer of the polymeremployed, and the internal cavity part 2 is completely devoid of thepolymer employed, as shown in FIGURES 1 and 2, whereas -any memberconstituting the conventional fibrous bundle is entirely filled up withthe material employed as shown in FIGURE 3, and the most desir-ablerange of the empty space cavity in terms of the aforesaid formula isfrom 30 to 60 percent.

The internal cavity of the filaments of this invention generallyconstitutes an elongated continuous space. In some instances, however,some laments exhibit an internal cavity having a discontinuous portion,but the existence of such discontinuous portions is in a quitenegligible order in view of theentire length of the elongated filaments.

According to the principles of this invention, a spinning mixture of aviscose and an aqueous Adispersion of a tetrauoroethylene polymer iscontinuously extruded at from 50 to 100 C. into -a setting mediumconsisting of sulfuric acid in the order of from 6 to 60 percent byweight. Alternatively, said spinning mixture may be extruded into asetting medium comprising sulfuric acid in the range of 6 percent orless by weight and a sulfuric acid salt in the order of at least 5percent by weight; or hydrochloric acid in the order of from 1 to 30percent by weight; or nitric acid in the range of from 1 to 50 percentby weight.

According to the principles of this invention, the average rate ofcavity formation reaches as high as 70 to 100 percent. The term cavityformation or cavity formation rate as employed herein is defined -asbelow:

Percent cavity formation rate:

For example, when a filament bundle given consists of all hollowfilaments as shown in FIGURE 2, the cavity formation rate thereof is 100percent, whereas the cavity formation rate of the conventional filamentbundle having no hollow filaments as shown in FIGURE 3 is 0 percent.

The tetrafluoroethylene polymers employed in this invention contain atleast 70 percent by weight of tetraiiuoroethylene ingredients. Saidpolymers include polytetrafluoroethylene and copolymers ftetrafluoroethylene with polymerizable materials, such ashexafluoropropropylene, trifiuorochloroethylene or vinyl chloride. Saidpolymers, further, must be of a molecular weight sufficient ly high tobe fiber-forming, and the higher the molecular weight, the betterresults are obtained, because the physical properties of the filamentsobtained are improved in accordance with a rise in the molecular Weightof the starting polymer.

The tetrauoroethylene polymers are employed in this invention in theform of aqueous dispersion prepared by emulsion polymerization or bydispersing the polymer in the form of particles in the aqueous medium.To the resultant dispersion there may be added a surface active agent,where necessary, and the dispersion is mixed with a viscose. Theresultant spinning mixture is very stable and there areformed nosubstantial precipitates of the high molecular weight susbtancesemployed. The cellulose content present in the spinning mixture must bein the order of from 0.4 to 8 percent by weight, preferably from l to 5percent by weight, and the tetraffuoroethylene polymer content presentcomprises from 70 to 99 percent by weight, preferably from 75 to 97percent by weight, of the combined weight of said polymer and cellulose.

In this invention, the composition ratio of the setting medium is animportant factor. The acid concentration of the setting medium varies inaccordance with the species of the acid employed. Sulfuric acid isemployed in a concentration ranging from 6 to 60 percent by weight,preferably from to 50 percent by Weight. There must be added a sulfuricacid salt in the order of at least 5 percent by weight when the sulfuricacid employed is in the order of 6 percent or less by Weight.Hydrochloric acid is employed in a concentration ranging from 1 to 30percent by weight, preferably from 4 to 20 percent by weight;

and nitric acid in the order of from 1 to 50 percent by weight,preferably from 3 to 40 percent by weight.

In this invention, the addition of a salt is imperative in the aforesaidinstance where the sulfuric acid employed is in .the order of 6 percentor less by weight. In other instances, however, said addition is notcritical. The salts added include metal salts of the acid employed, suchas alkali metal, alkali earth metal, magnesium, zinc, aluminum, chrome,manganese, copper, iron and the like metal salts, of which natrium saltand zinc salt are preferable. These salts are employed either singly orin mixture of 2 or more species.

The temperature applied to the setting medium which is also one of theimportant factors of this invention and rather higher than theconventional temperature, ranges from 50 to 100 C. The desirabletemperature is determined in accordance with the species of thecomponents constituting the setting medium. The desirable temperatureapplied to the sufuric acid coagulating bath ranges from 70 to 95 C.;the desirable temperature applied to the hydrochloric acid bath isbetween 50 and 70 C., and that applied to the nitric acid bath is in theorder from 55 to 90 C.

The spinning mixture containing any of the aforesaid polymers and aviscose is filtered, deaerated and extruded into the setting medium asspecified before. The hole diameter of the spinneret and the draftduring spinning vary over a wide range, but it is desirable toaccommodate these conditions so as to produce a filament having a crosssectional area of the polymer part forming the filament, not includingthe internal cavity space, in the order of not less than 5 105 cm?. Thefilaments obtained at this stage of coagulation are not sufficientlystrong, because particles of the tetrafiuoroethylene polymer employed isadhesively imbedded in the cellulose matrix.

The filaments obtained are subjected to the conventional coalescingprocedure as described in United States Patent 2,772,444 after washingand drying, comprising heating the filaments at a temperature higherthan the fusing point of the tetrafluoroethylene polymer employed.Polytetrafiuoroethylene, for instance, is subjected to a temperaturehigher than the transition temperature of 327 C., preferably to atemperature in the range of 350 to 450 C. The cellulose matrix isthereby decomposed, and the polytetra'fiuoroethylene employed coalesces.

Thus, the desired hollow filaments having from 5 to 90 percent emptyspace cavity are obtained in such a high cavity formation ratio as from70 to 100 percent. The resultant structures-are drawn and heat-treatedaccording to the conventional method where desirable.

The theoretical reasons for the formation of internal cavities withinthe body of the filaments of this invention are not necessarilyapparent. There are however employed a setting medium having anappropriate order of concentration in terms of the acid employed, and atemperature ranging from 50 to 100 C., so that the spinning mixtureextruded in the setting medium instantaneously coalesces, forming afast-hardening surface layer, and the internal portion of the resultantfilaments is forced to harden progressively and retardedly and attractedto the fasthardening surface layer, forming a hollow space or cavitywithin the body of each of the filaments obtained.

The hollow filaments of this invention possess excelled heat-resistanceand chemical stability and a low order of coefficient of friction. Thehollow cavities formed within the body of the laments impart a bulkinessto the filaments, reducing the cost per unit volume. The application ofpolytetrafiuoroethylene fibers or fabrics has hitherto been confined toa limited area despite their suitable properties, because the marketprice of said fibers or fabrics remained exceedingly high. Thestructures of this invention however are inexpensive in cost price, sothat the structures can be profitably employed as a desirable materialfor manufacturing packings, lubricating articles for bearings, filteringfabrics, safety clothings and the like chemicaland heat-resistantarticles. The excelled heatinsulating property and compressiveelasticity of the structures of this invention which cannot be achievedwith the conventional structures of polytetrafiuoroethylene, can furtherbe profitably utilized for the manufacture of articles requiring suchproperties, such as felt and the like.

In order to afford a fuller understanding of this invention, there areprovided preferred examples which are illustrative only and in whichpercent compositions and parts are all by weight.

Example I In an autoclave was placed a mixture of 0.13 gram of ammoniumpersulfate, 3.6 grams of ammonium perfluorooctanoate, and 36' grams ofparaffin in 1200 cc. of water. There were then introduced 600 grams oftetrafluoroethylene, and the polymerization reaction was carried out at60 C. under a pressure of 28 kg./cm.2, which pressure was maintained bycontinuous supply of tetrafiuoroethylene. When 600 grams oftetrafiuoroethylene were consumed, the reaction was suspended. To theresultant aqueous emulsion of a concentration of 33 percent ofpolytetrafiuoroethylene was added sodium dodecylbenzen sulfonate in theorder of about 3 percent on the basis of the weight of the polymer.

A wood pulp containing about 95 percent of a-cellulose, on the otherhand, was dipped for 2 hours in an aqueous solution containing 17.5percent of caustic soda at 20 C., compressed to the order of acompression ratio of 2.5, pulverized, and immediately xanthated with 65percent of carbon disulfide. The resultant xanthate was dissolved indilute caustic soda, producing a viscose containing 4 percent ofcellulose and 2 percent of caustic soda in total, and having apolymerization degree of 650.

With 1 part of the viscose thus prepared were mixed 2 parts of thepolytetrafiuoroethylene emulsi-on as specified above. The mixture wasthen filtered, deaerated, and extruded at 75 C. through a spinnerentcontaining 20 holes of 0.2 millimeter diameter into a coagulating bathconsisting of l percent of sulfuric acid, 20 percent of sodium sulfate,and 5 percent of zinc sulfate. The resultant filaments were washed withwater, sintered in the air heated to 375 C. for 100 seconds, andstretched 4 times the original length at 320 C. The filament bundle thusobtained measured 1162 denier .and displayed a cavity formation ratioiof 85 percent, a tenacity of 1.2 grams per denier, and an elongation of63 percent. An average empty space cavity of ythe resultant hollowfilaments is 32.5 percent.

lExample 2 A wood pulp containing about 92 percent of cellulose wasd-ipped at 20 C. in an aqueous solution containing 17 percent of causticsoda, compressed t-o the order of a compression ratio of 2.4,pulverized, raged at 30 C. for 48 hours, and xanthated with carbondisulfide added in the order of `60 percent on the basis of the weightof the tva-cellulose. The resultant xanthate was dissolved in alkali andaged at 25 C. for 40 hours, producing a viscose containing 8 percent ofcellulose and 6 percent of caustic soda, and having a polymerizationdegree of 300 and a :salt value `of 6.

The polytet'rauoroethylene emulsion of Example 1 was condensed to theorder of concentration of 60 percent polymer, and 1 par-t of theresultant icondensed emulsion was mixed with 1 pant of the viscose asprepared in the abo've specified manner. The mixture was filtered,`deaerated, and extruded at 75 C. through a spinneret having 20 holes of0.3 millimeter diameter into a coagulating bath containing percent ofsulfuric acid. The resultant filaments were washed with walter, sinteredin air heated to 380 C. for 80 seconds, and `stretched 4 times theoriginal length. The filament bundle thus Iobtained measured 254 denierand displayed a cavity formation rate of 98 percent, a tenacity of 1.41grams per denier, and an elongation of 40 percent.

Example 3 A mixture of 2 parts of the polytetrafiuoroethylene emulsionof Example 2 and 3 parts of the viscose of Example 1 was filtered,deaerated, an-d extruded at 50 C. through a spinneret containing 20holes of 0.-1 millimeter diameter linto a setting medium consisting of10 percent of hydrochloric acid. The resultant filaments were washedwith water and heated at 370 C. for 100 seconds and stretched 5 timesthe original length at 340 C. The filament bundle thus obtaineddisplayed a cavity formation rate lof 98 percent.

Example 4 The spinning mixture of Example 1 was extruded int-o acoagulating `bath containing 4 percent of nitric acid and 6 percent ofsodium nitrate at 80 C., and the resultant filament were treated underthe same conditions as employed in Example 1, producing a filamentbundle displaying a cavity formation rate of '100 percent. An averageemp-ty space cavity of the resultant 'hollow filaments lwas `43.0percent.

Example 5 A spinning mixture was prepared by adm-ixing 1 part of a 30percent aqueous ldispersion of 4:1 molar copolymer oftetrafiuoroethylene and hexafiuoropropylene with 1 part yof viscoseldescribed in Example 1, and the mixture was extruded at 60 C. into .acoagulating bath cont-aining 20 percent of nitric acid. The resultantfilaments were washed with water and s'intered in the air heated fto 350C. for 40 seconds, producing a -lament bundle having a cavity formationrate of 96 percent.

In view of the foregoing, it will be apparent to those who are skilledin the art that there may be made various changes and modifications inthe invention without departing from the scope and spirit of the same.It is accordingly requested that the invention be interpreted ratherbroadly and not limited to the specific embodiments of the same exceptas defined in the appended claims.

We claim:

1. A hollow filament consisting of a tetrauoroethylene polymer layer andan empty space cavity within, said empty space cavity comprising from 5to 90 percent in terms of secshxw0 References Cited UNITED STATESPATENTS 2,772,444 12/ 1956 Burrows et al. 264--127 2,855,321 10/1958Bachlott 264-188 X 3,194,861 7/1965 Bley 264-191 X ROBERT F. WHITE,Primary Examiner.

T. J. `CARVIS, Assistant Examiner.

